The invention relates to a communication device, a control device and a communication system.
In a ubiquitous computer scenario, such as a “Smart House” (that is to say a house with intelligent control of individual domestic appliances), there are frequently some appliances with high computation power and a large memory space, such as a desktop computer, a laptop or a PDA (Personal Digital Assistant). Such appliances are subsequently also referred to as terminals. Terminals can be used to monitor and control small devices which have been integrated into this computer environment, such as thermometers, pressure sensors or switches.
In the scenario under consideration, these small sensors or actuator devices can be coupled to the terminals so as to be able to communicate, by connecting wireless radio frequency transceivers to the sensors or actuator devices (for example an active RFID tag (Radio Frequency IDentification tag). Such sensors and actuators which have a radio frequency transceiver or can be fitted to a radio frequency transceiver are subsequently also referred to as transceivers. Normally, a terminal is either operated using an electrical power line (as in the case of a desktop computer, for example) or it can be provided so as to be rechargeable (as in the case of a laptop or PDA, for example). This means that the admissible power requirement for such terminals is typically not seriously limited. In other words, the power supply for terminals is uncritical in many cases.
By contrast, transceivers are usually powered by small batteries, and it is inconvenient for a user to replace such batteries frequently. This is because of the large number of such transceivers in a “Smart House”, inter alia. Power consumption is therefore one of the critical technical factors when developing transceivers. In a transceiver apparatus, the radio frequency transceiver is one of the greatest consumers of power.
In addition, methods known from the prior art are described which a control unit (such as a laptop computer) can use to access communication devices, such as RF transceivers, or which can be used for communication.
Without a specific access scheme, most RF transceivers need to be kept continuously in a reception mode and are therefore continually awaiting possible arrival of a packet of data from the terminal. This continual operation of an RF transceiver in an activated state is extremely inefficient as regards the power consumption of the transceiver, since the power consumption of an RF transceiver in a reception mode is very high, which means that a battery can be drained within a few days.
The prior art includes known methods which can be used to reduce the power consumption in comparison with continual activated operation of an RF transceiver.
One known solution is “Media Access Control (MAC) Layer Duty Cycle Scheduling”. The “IEEE802.15.4 Low-Rate Wireless Personal Area network” Standard described in LAN/MAN Standards Committee of the IEEE Computer Society. IEEE Standard for Information Technology, Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs), October 2003, is one example of this. In a master-slave star topology network, the master (for example a terminal) periodically transmits a signal frame (beacon). The beacon contains synchronization information and indicates the slave nodes (for example transceiver) with which the master which is to communicate in a particular period. The slaves awake periodically to detect a beacon. If there are pending packets available for a particular slave, which is indicated in a beacon, or if a slave has packets for the master then the slave can remain activated (“awake”) for the rest of the period of communication. If there is no packet, the slave changes to a power-saving sleep mode until the next beacon is transmitted by the master. In this way, the slave awakes its MAC module and its RF transceiver in each period for the length of a beacon. If the ratio of beacon length to period, defined as the “duty cycle”, is very low (for example 1%), the power requirement for the slave RF transceiver falls by a factor of one hundred under the constraint that no useful data are transmitted. However, the drawback of this mechanism is that only a star topology with a master supports this scheme, since a plurality of masters produce a plurality of beacons, which would confuse a slave which detects all the beacons. Another drawback of this method is that both the MAC module and the RF transceiver need to be awoken in order to receive and forward the beacon, specifically regardless of the presence of transmitted useful data.
Another known method is to design a wakeup-by-signal RF transceiver. This means that when a terminal with a sleeping transceiver wishes to communicate, it emits a specific wakeup signal which can wake up the sleeping transceiver. In this way, a transceiver is awoken only upon request instead of being awoken periodically, so that the power consumption can be reduced in comparison with the method in which the sleeping transceiver is awoken periodically. This method is also not limited to one particular network topology.
Ideally, the transceiver remains passive for detecting the wakeup signal. However, a fully passive RF transceiver can be produced only with a high level of design complexity, and to date there is no mass-produced product which satisfies this functionality.
In addition, the “Preamble Sampling” scheme disclosed in El-Hoiydi, A “Aloha with Preamble Sampling for Sporadic Traffic in Ad Hoc Wireless Sensor Networks”, CSEM, 2002 has been proposed which combines the two previously described methods with one another. On the basis of this scheme, each RF transceiver awakes itself for a short time, during which the RF transceiver takes a Received Signal Strength Indicator (RSSI) as a basis for detecting whether the channel is in use. If the channel is in use, the RF transceiver awakes other components of the apparatus so as to detect the signal on the channel. Otherwise, it remains in a sleeping state. This detection method can be referred to as “scanning”. To send a packet to such an RF transceiver, that is to say to scan the channel, a terminal must first of all transmit a WakeUp Preamble (WUP) with a length which is greater than the scanning period, so that it is certain that the scanning RF transceiver can detect the preamble and can waken the components of its apparatus to receive the packet which comes after the preamble. This scheme is referred to as a WUP scheme. The WUP scheme is not a genuine wakeup-by-signal scheme, since the RF transceiver still has a period which it uses to awake itself. However, the time required for scanning is greatly shortened in comparison with that for monitoring a signal frame in line with the IEEE802.15.4 standard. In addition, the scanning by using RSSI can be performed by the RF transceiver alone without the aid of other modules. Another advantage is that the WUP scheme can be used in any desired topology, such as a star mesh topology with a plurality of masters (terminals) or in a peer-to-peer topology without a master.
One drawback of the WUP scheme is its low efficiency with regard to the maximum attainable data throughput, on account of the transmission of long preambles. The length of the preamble is dependent on the scanning period, which is frequently in the order of magnitude of several 100 ms. However, a packet length in a wireless sensor network is normally fairly short and requires only a few 10 ms for transmission. Consequently, preambles produce a large part of the traffic on this channel, which drastically reduces the maximum attainable data throughput for actual useful packets. This is the reason why the WUP scheme is suitable only for networks with sporadic traffic in which the maximum attainable data throughput is not a critical factor. Another drawback of the WUP scheme is the additionally required power which is consumed by sending long WUPs.
Therefore, what is known as a WiseMAC protocol has been proposed which is based on the WUP scheme and in which the length of the WUP is shortened, see El-Hoiydi, A et al. “Poster Abstract: WiseMAC, an Ultra Low Power MAC Protocol for the WiseNET Wireless Sensor Network”, CSEM SA, November 2003. On the basis of this protocol, each apparatus stores the scanning schemes from its neighbors, which are attached in “piggyback” form to the last received packet from the neighbor. This allows a short WUP to be used in order to waken a neighbor on the basis of the stored scheme. However, inaccuracies in the clock signals in the apparatuses cause a time drift to arise between the apparatuses, so that the length of the preamble needs to be extended in order to compensate for the time drift if the wakeup scheme of the destination nodes has already been received a considerable time ago.
The WiseMAC protocol allows the use of a much shorter preamble, which firstly increases channel capacity and secondly saves power which is required for sending a long WUP.
The WiseMAC protocol therefore allows a shorter preamble to be used in comparison with the conventional long WUP. However, the long WUPs are still needed in many cases. Firstly, the very first packets between the neighbors should be sent using a long WUP. Secondly, the compensation for the time drift can make a short WUP much longer, so that the length can be increased up to the length of the scanning period. In addition, what is known as a broadcast needs to use a long WUP in order to waken all the neighbors. Apart from the broadcast/multicast traffic in the topmost layer, broadcasts exist in widespread form in a network with mobile devices during network discovery, handshaking, multihop routing etc. A large quantity of long WUPs is therefore unavoidable, which wastes power and channel capacity.
The text below refers to FIG. 1 and FIG. 2 to describe a WakeUp Preamble scheme 100 based on the prior art.
FIG. 1 illustrates a transmitter scheme 110, that is to say a time sequence of states as adopted by a transmitter. In addition, a receiver scheme 120 is illustrated, that is to say a time sequence of signals as adopted for RF scanning by a reception device. In addition, an FPGA scheme 130 is illustrated, that is to say the time sequence for the operating state of an FPGA (Field Programmable Gate Array, a programmable logic circuit, that is to say a structure including configurable logic chips, each of which can be used, by way of example, as AND, OR, NOT, XOR, etc.). In addition, a microcontroller scheme 140 illustrates the time sequence for the operating states of a microcontroller.
The reception device, the FPGA and the microcontroller are coupled to one another and form a communication device (for example an active RFID tag) which is provided so as to be able to communicate with a control unit (for example a reader).
As can be seen from the transmitter scheme 110, the transmitter sends a data frame 112 after it has sent a long WUP 111 (WakeUp Preamble). As can be seen from the receiver scheme 120, the RF receiver is alternately in a deactivated state 121 or in an activated state 122. The activated state 122 contains an RF setup state 123, in which the receiver is prepared for an activated state, and a DRD state 124 (data rate detection), in which the receiver is ready to receive. In this reception state, the transceiver evaluates the received signal, e.g., using the RSSI and/or the incoming data rate, with the aim of detecting the WUP. If the receiver is in an activated state 122 during transmission of the WUP 111, the RF transceiver detects this preamble 111 and uses a wakeup command 150 to waken the FPGA, previously in a power-saving state. This changes the FPGA from a deactivated state 131 to an activated state 132, the FPGA briefly adopting an FPGA setup state 133 between the deactivated state 131 and the activated state 132. The FPGA then switches the RF transceiver from a self-polling mode (that is to say a mode in which the transceiver switches itself to and fro between an activated state 122 and a deactivated state 121) to a reception mode for receiving the data frame 112. This is done by a wakeup command 151. After such a wakeup command 151, the RF transceiver changes from the deactivated state 121 via an RF setup state 123 to the reception mode 125. The microcontroller is then woken up in order to process the frame. For this, the microcontroller is transferred from a deactivated state 141 to an activated state 142, specifically by a wakeup command 152 from the FPGA. Between the deactivated state 141 and the activated state 142, the microcontroller briefly adopts a microcontroller setup state 143.
The RF transceiver, the FPGA and the microcontroller have a respective setup delay 123, 133 or 143. The length of the WUP 111 is calculated such that it is sufficient even for a worst case scenario in which the start of the WUP 111 just misses a DRD interval 124. In addition, the setup delays 123, 133, 143 of the components need to be taken into account.
The WUP scheme 100 based on the prior art has the drawback that if the RF transceiver detects a valid data rate at the start of the WUP 111 then it wakens the FPGA and the FPGA controls the RF transceiver such that it changes to the reception mode. The RF transceiver and the FPGA have to wait a relatively long time before they receive the data frame 112, as illustrated in the dashed loss area 201 in FIG. 2. Since this waiting time is on average approximately half of the scanning period, which may be in the region of several 100 ms, a large quantity of electrical power is consumed by the RF transceiver and by the FPGA in the loss area 201. This means that the methods known from the prior art for communication between a communication device and a control device are not suitable to be operated with a sufficient low power requirement.
DE 102 13 114 A1 discloses a method for the periodic radio transmission of measurement data from a plurality of measurement devices on a common receiver. To reduce the current drawn in the receiver, it is proposed that the receiver be put into reception standby only in a time window when transmission of the measurement data from a particular measurement device is expected.
DE 101 36 757 C1 describes a method for operating a radio system in which receivers are respectively allocated a characteristic of a radio signal and a receiver is changed from a standby mode to an operating mode when it receives a radio signal which has the characteristic which is associated with it. The characteristic is a particular data rate for the radio signal, for example.
DE 199 52 840 A1 describes the transmission of digital data through the transmission of data packets from a transmitter to one or more receivers, the transmitter sending waiting packets from which the receivers can determine a waiting time up until the transmission of data packets. For the duration of the waiting time, the receivers deactivate their reception device.
U.S. Pat. No. 5,918,170 describes a method for operating a mobile radio communication network with a set of mobile radio terminals. A control channel is repeatedly used to send a frame structure, and the mobile radio terminals are controlled such that they leave a sleep mode at particular intervals of time and receive the frame structure at regular intervals.
WO 2004/030251 A1 discloses a communication system in which a message which has a message portion indicating the time for the transmission of a later signal is sent from a transmitter to a receiver.
For these and other reasons, there is a need for the present invention.